A discussion is presented of daytime sky imaging and techniques that may be applied to the analysis of full-color sky images to infer cloud macrophysical properties. Descriptions of two different types of skyimaging systems developed by the authors are presented, one of which has been developed into a commercially available instrument. Retrievals of fractional sky cover from automated processing methods are compared to human retrievals, both from direct observations and visual analyses of sky images. Although some uncertainty exists in fractional sky cover retrievals from sky images, this uncertainty is no greater than that attached to human observations for the commercially available sky-imager retrievals. Thus, the application of automatic digital image processing techniques on sky images is a useful method to complement, or even replace, traditional human observations of sky cover and, potentially, cloud type. Additionally, the possibilities for inferring other cloud parameters such as cloud brokenness and solar obstruction further enhance the usefulness of sky imagers.
Several features that can be extracted from digital images of the sky and that can be useful for cloud-type classification of such images are presented. Some features are statistical measurements of image texture, some are based on the Fourier transform of the image and, finally, others are computed from the image where cloudy pixels are distinguished from clear-sky pixels. The use of the most suitable features in an automatic classification algorithm is also shown and discussed. Both the features and the classifier are developed over images taken by two different camera devices, namely, a total sky imager (TSI) and a whole sky imager (WSC), which are placed in two different areas of the world (Toowoomba, Australia; and Girona, Spain, respectively). The performance of the classifier is assessed by comparing its image classification with an a priori classification carried out by visual inspection of more than 200 images from each camera. The index of agreement is 76% when five different sky conditions are considered: clear, low cumuliform clouds, stratiform clouds (overcast), cirriform clouds, and mottled clouds (altocumulus, cirrocumulus). Discussion on the future directions of this research is also presented, regarding both the use of other features and the use of other classification techniques.
Abstract. The most extensive study of frequency, magnitude and sky properties associated with UVB enhancement is reported. The first integrated automatic sky camera and irradiance measurement system was used for simultaneous data collection over the solar zenith angle range of 4.2 to 64.3 ø. It is reported that 3 % of UVB irradiance measurements collected at intervals of 6 minutes, over a one-year period, were cloud enhanced. It was found that 85 % of possible UVB enhancements occurred for solar zenith angles ranging from 40 to 63 ø. The study was undertaken during September 1997 to August 1998 at Toowoomba, Australia (27.6 øS, 151.9 øE). The maximum UVB enhancement was found to have an irradiance of 8 % greater than that of an equivalent clear-sky. A mechanism for UVB enhancement for cases of solar obstruction by cirrus cloud is postulated. Analysis showed that 86 % of enhancement occurred under hazy/cirrus skies.
The optical properties of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) film have been characterised in order to develop an alternative method for UV dosimetry with a focus on long-term human exposure measurements. The dynamic range of PPO film was found to extend to 2 MJm -2 of broadband UV exposure independently of film thickness, providing an exposure range of roughly four summer days at subtropical latitudes. The sensitivity of the film to UV exposure was positively related to film thickness in the 20 to 40 μm range. Films of 40 μm thickness proved to be the most suitable for long-term human UV exposure measurements.The temperature independence of the response of 40 μm PPO film was established from 1.5°C to 50°C within a dosimeter response uncertainty of 6.5%. Dose-rate independence was also demonstrated within 8% of the mean dosimeter response. The spectral response approximates the CIE erythemal action spectrum between 300 and 340 nm, with a peak response at 305 nm. A large deviation from this action spectrum was observed at shorter wavelengths. Investigation of the angular response in both the azimuth and altitude planes showed a cosine error of less than 6.2% between 0° and 40°, and did not exceed 13.3% at any angle greater than 40°. These results indicate that PPO film satisfies the requirements for use as a UV dosimeter, and may be employed in long-term human exposure measurements.
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